#include <cstdio>

using namespace std; 

#define N 	11
#define TN	(1<<N)

char s[TN]; 
int n; 

char show(int a, int b)
{
	int m;
	char left=0, right=0, r=0; 
	
	if (b-a>1) {
		m = (b+a)/2; 
		left = show(a, m); 
		right = show(m, b); 
	} 
	if (left && right) {
		r = (left==right && left!='F') ? (left=='I' ? 'I' : 'B') : 'F'; 
	} else {
		r = (s[a]=='0' ? 'B' : 'I'); 
	}
	printf("%c", r); 
	return r; 
}

int main()
{
	//freopen("p1087.in", "r", stdin); 
	scanf("%d\n", &n); 
	scanf("%s", s); 
	//printf("n=%d\n", n); 
	//printf("s=%s\n", s); 
	show(0, (1<<n)); 
	printf("\n"); 
	return 0; 
}


